Recovering Payments Made By Mistake
The doctrine of equitable subrogation applies when one person makes payment on behalf of another. There need not be a contractual duty to make the payment. Howevever, equitable subrogation will not apply if the payment is made as a "volunteer." So what is a volunteer? The Sixth Circuit Court of Appeals looked at that question this month, at least under Michigan law, in Fed. Ins. Co. v. Hartford Steam Boiler Inspection and Ins. Co.:
A “volunteer” has been defined as “one who intrudes himself into a matter which does not concern him, or one who pays the debt of another without request, when he is not legally or morally bound to do so, and when he has no interest to protect in making such payment.” Further, a “payment is not voluntary when made under compulsion, under a moral obligation, in ignorance of the real state of facts, or under an erroneous impression of one’s legal duty.”
The part worth noting is that last bit: if a person pays because of their own subjective mistake of fact or law, they may rely on equitable subrogation to recover their payment. The Court's opinion says nothing about requiring the mistake to be objectively reasonable.